To avoid such a switch being destroyed by excessive current surge due to a short circuit, it is known to monitor the current flowing through the switch and to turn the switch OFF as soon as the value of the current exceeds a predetermined threshold, and then to switch the switch back ON after a time delay while being ready to switch it back OFF again quickly if the excessive current persists.
Such a protection circuit should preferably present the following features:
in order to avoid switching OFF the switch unnecessarily, it should ignore brief surges ("transients") that may appear in the load, for example when engaging a capacitive load or in the event of interference; PA1 in order to avoid running the risk of damaging the switch, the lengths of time during which it is switched beck ON again should be as short as possible if the short circuit persists; and PA1 in order to return the switch to its normal state as quickly as possible on disappearance of the short circuit state, the time delay should be short. PA1 means for obtaining an electrical parameter representative of the current flowing through the switch; PA1 means for generating a variable voltage that varies in a direction which depends on the position of said parameter relative to a predetermined reference; PA1 first and second threshold detectors receiving said variable voltage and respective first and second thresholds; and PA1 memory means contacted to the first and second threshold detectors to switch the switch OFF and ON when the variable voltage crosses the second or the first threshold respectively, on leaving the range defined by said thresholds; PA1 the second threshold being reached only after said parameter has crossed the predetermined reference for a certain length of time, thereby providing a degree of immunity against interference from transients.
It can thus be seen that there is a difficulty because these requirements are fundamentally contradictory: to avoid taking account of transients, the sensitivity of the circuit needs to be low, which means that the switch is switched back ON again for longer periods and at a lower repetition rate.
The present invention seeks to provide a protection circuit that satisfies the above conditions while being extremely simple and cheap.